Photography
The enduring image
In the commercial battle between digital and analogue photography, physics
eventually prevailed. Here, Mike Ware reveals how chemistry shaped the history of
photographic images and argues that it still has a part to play
ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY
For 150 years, the science
of chemistry and the art of
photography have been entangled in
a close relationship. A partnership
that, according to a 1858 edition of
the literary magazine Athenaeum,
was not entirely equal: ‘The artists
go on boldly and are not afraid to be
chemists; the chemists gain courage
and long to be artists.’
However bold the artists,
without a theoretical guide it was
experimental chemistry – trial
and error – that led the way. In
1840, the polymath Henry Talbot
happened upon the discovery that
gallic acid (3,4,5-trihydroxybenzoic
acid) could ‘bring out an invisible
dormant picture’ which had been
briefly projected upon a layer of
silver iodide in a camera obscura.
This phenomenon – now called
the development of a latent silver
image – was, at the time, utterly
inexplicable. Photography was
a ‘black art’. Talbot’s friend and
eminent colleague in the Royal
Society, Sir John Herschel, jestingly
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called the effect ‘really magical’,
and asked, ‘surely you deal with
 Photography,
the naughty one?’ Devilry aside,
once seen as a
the true explanation of the latent
black art, developed
image in silver halides would have
through chemical
to wait a century, until the quantum
experimentation
theory of solid state chemistry was
 Historically, all
established.
cameras used the
But the successful partnership
photochemical
of chemistry and photography
properties of silver halide was finally disrupted late in the
crystals but other metal
20th century by the rise of digital
salts can give different
technology. The convenience of
tones and colours when
electronic image manipulation
printing the negative
using personal computers moved
 Although digital
the entire scientific basis for the
technology has all
practice of photography from
but won the battle for
chemistry to physics. Today,
commercial photography, images are recorded on chargechemical printing still
coupled devices and printed
offers beautiful effects
piezoelectrically by ink-jet printers.
and permanent images
Multitudes of photographers
have now deserted their wet (and
sometimes smelly) darkrooms, in
favour of the dry comforts of their
desktops. Moreover, many who
were unable to print their own
photographs in the traditional
way can now do so with ease.
For most purposes, the analogue
silver image on cellulose paper or
polymer film has been replaced by
a computational abstraction – the
string of binary digital code. Pixels
substitute for silver grains, and the
photo CD serves instead of the roll
of negative film. Photography has
also fallen in with a new partner
– the internet. The processing and
electronic transmission of pictorial
information within our culture has
now reached unparalleled speed
and ease. Which raises the question:
Sir John Herschel:
is there any place left in today’s
inventor of siderotype
photographic technology for new
photographic printing
chemical science?
In short
Seeing the light
Talbot made his first camera
negatives in 1835 on his ‘photogenic
drawing paper’ using silver chloride.
Rather than ‘develop’ these, he
‘printed them out’ entirely by the
action of light alone. This was not
an efficient process – it took him
about an hour’s exposure with
the lens open (at a wide aperture
of f/4) to record those first weak
images with his tiny cameras that
his wife Constance referred to as
‘mousetraps’.
But the chemistry improved
and the cameras grew larger.
Talbot’s subsequent discovery
of the latent image improved the
sensitivity [to light] by a factor of
about a hundred, so his ‘calotype’
process of 1840 required camera
exposures of a minute or less. This
made photographic portraiture
possible for the first time. His
negative–positive process on paper
set photographic science on the
road to success. By the middle of the
20th century, the effective quantum
yield (the reaction produced by
one photon of light) from gelatinesilver halide camera emulsions had
improved dramatically. A typical
camera exposure to an average
subject was just 1/100 of a second at a
much smaller aperture (f/8).
This marriage of science and
art was founded on the unique
photochemical properties of
silver halide crystals suspended
in a gelatin film, the misnamed
‘emulsion’ of photography. Taken
up by George Eastman’s Kodak
Company in the US, the technology
became embodied in a large and
profitable industry, serving a
huge constituency of users, both
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PETER BARGH, EPHOTOZINE.COM
amateur and professional. This
commercialisation gave analogue
photography the benefit of rapid
technical development towards the
pinnacle of optical recording.
The history of photographic
science is studded with chemical
triumphs. An early example is
Herschel’s use in 1839 of sodium
thiosulphate to ‘fix’ the silver image
by complexing and dissolving out
the excess insoluble halide. In the
1920s, Samuel Sheppard at Kodak
discovered that traces of thioorganics, such as allyl isothiocyanate,
were responsible for the enormous
increase in the photosensitivity of
the emulsions made with bovine
gelatine. As he put it, ‘if cows didn’t
like mustard [plants], there wouldn’t
be any movies at all.’
In the 1980s, the technology
of ‘tabular grain’ crystal growth
brought the quantum efficiency of
developed silver halides even closer
to the theoretical limit. The latent
image in a single silver halide crystal,
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which renders it entirely reducible
to silver by ‘developing’ agents such
as aromatic amines and phenols,
consists of a ‘sensitivity speck’: a tiny
cluster of about four silver atoms.
Printing in metal
Most photographic practice entails
two separable stages. First, acquire
the image information, and then – at
some later stage – print it in a visually
acceptable form. For the nearinstantaneous capture of analogue
negative images in the camera, there
has never been any viable chemical
alternative to the uniquely fast
‘emulsion’ of gelatin-silver halide.
But making a positive print from
a camera negative is much less
exacting: it is no great disadvantage
to use lengthy exposures and intense
ultraviolet printing lights, which
open the door to a whole range
of photochemical systems, less
sensitive than silver halides.
For print-making there are many
alternatives to the ubiquitous
Chemical
experimentation lies
behind the magic of the
darkroom
‘Because cows
like mustard
plants, we
can go to the
cinema’
silver technology, most of which
were discovered in the dawn of
photography. In 1839, the Scot
Mungo Ponton noted a light-induced
colour change in dichromates coated
on paper, corresponding to the
photoreduction of chromium(vi).
Ultimately, chromium(iii) results,
which can insolubilise a colloidal
layer binding a stable pigment.
This technique was used to bind
pigments in the ‘carbon process’,
commercialised by the Londonbased Autotype Company in
1866, and in the painterly ‘gumbichromate’ printing process.
Both processes provide images as
permanent as the artists’ pigments
they employ.
The seminal discovery of
‘siderotype’, or iron-based
photographic printing, was made by
Sir John Herschel in 1842: iron(iii)
in certain carboxylate salts can be
photoreduced to iron(ii), which is
then reacted to make permanent
images (see box, p64): either
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Photography
Siderotype chemistry
Siderotype printing relies on the photochemistry of iron (iii) carboxylate, and its
subsequent reactions with gold. First, trisoxalatoferrate(iii) anion undergoes
an internal redox reaction induced by light with a wavelength of about 365 nm:
hν + 2[FeIII(C2O4)3]3– → 2[FeII(C2O4)2]2– + 2CO2 + C2O42–
The resulting iron(ii) oxalato complex can reduce noble metals from their salts:
[AuCl4]– + 3[FeII(C2O4)2]2– → Au + 3[FeIII(C2O4)2]– + 4Cl–
But gold(iii) is sufficiently oxidising to react with oxalate anion:
2[AuCl4]– + 3C2O42– → 2Au + 6CO2 + 8Cl–
so the constituents of the sensitiser cannot be pre-mixed.
similar to those highly-esteemed
media for other works of art on paper
– etching, engraving, and mezzotint
– the first printing method to give
tonal qualities. Art connoisseurs
appreciated the ‘platinum look’ and
its permanence, contrasting it with
the glossy, brown silver-albumen
prints of the day, which also suffered
from the vulnerability of silver to
sulfiding, and consequent fading
– a common fate of photographs
exposed to the polluted atmospheres
of Victorian England.
A night at the palladium
But the triumph of platinotype
was short-lived, thanks to the
discovery by Friedrich Wilhelm
Ostwald in 1902 of the catalytic
power of platinum in the oxidation
of ammonia to nitric acid. In 1916,
at the height of the Great War, the
combatant governments banned
the use of platinum for frivolous
purposes such as photography
and jewellery, and elevated the
metal to the status of a strategic
material, reserved for making nitrate
explosives. Willis immediately
devised a palladium printing paper
as a substitute.
His Platinotype Company
survived until 1937. Its ultimate
demise was due to the limitation
suffered by all the iron-based
processes: the necessity for printing
by contact, using a same-sized
negative, to allow a sufficient
throughput of light. The growing use
‘Gull, South Ronaldsay’
- printed as a cyanotype
(left), palladotype
(centre) and chrysotype
(right)
of miniature cameras that recorded
negatives onto rollfilm meant that
enlargement by projection became
the norm, requiring a sensitivity
which only silver photography can
offer.
But during the late 1970s,
a rebellion began against the
uniformity of the commercial silvergelatin enlarging papers provided
by the market-driven industry. A
few photographic artists started
hand-sensitising their own papers,
leading a contemporary renaissance
in alternative photographic printing,
using long-forgotten 19th century
processes. So platinum-palladium
printing has regained its place as
a minority practice for an élite
band of photographers. In this era
of mass production, handmade
photos in platinum metals carry a
certain cachet, and can claim greater
permanence, than the silver or inkjet prints on commercial materials.
Forgotten colour
Throughout this history, platinum’s
neighbour gold was ignored as a
potential image substance in its own
right, although the metal had been
used extensively from the earliest
days of photography to tone and
stabilise silver images by partial
electrochemical displacement.
Herschel’s pure gold printing
process – his chrysotype of 1842
– never gained admission to the
photographic repertoire, and by
1900 it had been entirely discounted
as ‘dead and buried’. The problem
lay with the chemistry: the
simplistic 19th century method
used the easily-available salts of
tetrachloroaurate(iii). This could
not be mixed with the iron(iii)
carboxylate sensitiser, owing to
redox instability which causes the
premature precipitation of gold
metal. Instead, the gold salt had to
be employed in a developer bath,
which was uneconomic. The answer,
which I discovered in 1986, was
complexation and reduction to
gold(i), to provide an efficient and
MIKE WARE
by the reduction of noble metal
salts such as silver (argentotype),
gold (chrysotype), or mercury
(celaenotype).
The iron(ii) can also be coupled
with hexacyanoferrate(iii) to make
the pigment Prussian blue (iron(iii)
hexacyanoferrate(ii)). This was
Herschel’s cyanotype process, which
became the first commercially
important method of reprography
– the blueprint. Entirely organic
photosensitive substances were also
pressed into service, some literally:
Herschel extracted coloured
anthocyanins from the flowers of
his Kentish garden to serve in his
‘anthotype’ dye-bleaching process.
He also attempted to make prints
in platinum by his iron-based route,
but was unsuccessful. A working
platinotype process had to await the
research and enterprise of William
Willis, who made the key discovery
in 1873 that it was necessary to use
the (then uncommon) platinum(ii)
salts, rather than platinum(iv) salts,
for a successful reduction. Willis
marketed his platinotype paper in
1879, although it took him twenty
years to perfect the process. The
platinum print became widely
acclaimed for its permanence and
artistic effect. By 1900 the salons of
photography were exhibiting more
platinotypes on their walls than any
other process. The medium displays
pleasing characteristics: a totally
matte paper surface and subtle grey
tonal scale, with an aesthetic appeal
‘By 1900 the
salons of
photography
were exhibiting
more
platinotypes
on their walls
than any other
process’
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The new chrysotype process
This process relies on the ligand 3,3'-thiodipropanoic acid, S(CH2CH2COOH)2,
used in the neutralised form of its disodium or diammonium salts, which are
highly soluble in water. The reaction of this thio-ether (written below as SR2)
with tetrachloroaurate(iii) takes place in three steps:
i) Coordination to gold(iii): [AuCl4]– + SR2 → [AuCl3SR2] + Cl–
ii)Reduction to a two-coordinate gold(i) complex by a second molecule of
ligand: [AuCl3SR2] + SR2 + H2O → [AuClSR2] + OSR2 + 2H+ + 2Cl–
iii)Excess ligand may displace the chloride ion from the gold(i) complex:
[AuClSR2] + SR2 → [Au(SR2)2]+ + Cl–
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MIKE WARE
economic mixed sensitiser (see box,
below). The new chrysotype process
is a close analogue of the celebrated
platinotype, but brings one exciting
extra benefit: colour.
Since the Middle Ages, it has been
known that gold salts can impart rich
ruby reds and purples to ceramic
glazes and glasses. Purple of Cassius,
discovered by the proto-chemist
Johann Rudolf Glauber in 1656, is
now known to be nanoparticle gold
in a matrix of tin(iv) hydroxide.
The origin of this colour was first
established by Michael Faraday
in 1856 as due to the formation of
highly dispersed gold, but it was not
until 1904 that Gustav Mie explained
its visible absorption spectrum with
a classical scattering theory based
on the application of Maxwell’s
equations to spherical metallic
particles. The strong absorption
band of nanoparticle gold in the
visible region around a wavelength
of 520nm is due to a surface plasmon
resonance – a collective oscillation
of the delocalised conduction
electrons – and its wavelength is
dependent, among other things, on
the radius of the metal nanosphere.
Thus the colour of gold images
may be tuned by reactions yielding
nanospheres of differing sizes. If the
particles are ellipsoidal, rather than
spherical, two absorption bands are
generated and green gold can result.
These developments have provided
creative photographers with a new
palette of non-literal or ‘surreal’
colour to enhance the expressiveness
of their imagery, while also forging
an invigorating new link between
any prevailing computer technology
the graphic arts and the burgeoning
ensures that, providing they are well
science of nanotechnology.
prepared, they will remain humanly
readable forever. It is hard to see
Enduring images
how machine-dependent, digitallyAnalogue photographs – negative or encoded images can ever lay claim
positive – are repositories of pictorial to such robustness and accessibility.
information in a form easily handled The end of the traditional
and stored as permanent, flat objects. photographic negative and printed
Their complete independence of
record would prove a great loss
Chrysotypes like
‘Il Salvatore, Noto
Antica, Sicily’, show good
tonal range and surface
quality. A variety of
colours can be produced
depending on the size of
the gold particles
to many historical photographic
archives.
Although ink-jet printer pigments
are improving in their endurance,
some photographers prefer a hybrid
practice. They employ digital
techniques to ink-jet print large
negatives onto transparent film
(which may well prove ephemeral)
but then use them to print positives
in gold or platinum upon a substrate
of the finest cellulose paper. Such a
print can endure for a millennium.
This union of the noblest metals with
the commonest organic substance on
earth also provides the artists with a
satisfying embodiment of being true
to their materials.
Mike Ware is honorary fellow in
chemistry, University of Manchester
Further reading
http://www.mikeware.co.uk/
M Ware, Gold in photography. Brighton: Ffotoffilm, 2006
M Ware, The chrysotype manual. Brighton:
Ffotoffilm, 2006
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